The present invention relates to an anit-theft device which is associable with the read and write elements of a microprocessor operatively associated with a protected memory such as is described in the above-referenced parent application Ser. No. 936,694, now U.S. Pat. No. 4,211,919, granted July 8,1980.
In the parent application there is described a portable data carrier for the storage and processing of data, characterized in that it comprises a microprocessor associated with at least one programmable read-only memory, the microprocessor comprising elements for reading from and writing in parts of the programmable read-only memory as well as elements for the reception or transmission of data to be written in or read from the programmable read-only memory. The data carrier is moreover characterized in that the read and write elements comprise first means for recognition of an enabling key associated, on the one hand, with second means for storing an error bit in one zone of the programmable read-only memory when the key is incorrect and, on the other hand, with third means for storing an access bit in another zone of the programmable read-only memory when the key is correct.
As described in the parent application, it is necessary in certain applications to protect access to a permanent memory PROM by an access key associated with certain rights of reading and writing in specific zones. The prior art is abundantly provided with devices enabling the dissuasion of would-be defrauders who make several attempts to discover the access key to a memory. Amongst these devices the best known, without a doubt, is one which permanently stores the key errors, and blocks operation when the number of errors exceeds a predetermined limit. However, this device is inefficient if it is utilized alone. In fact, the observation of the variations of writing current in the memory enables the accurate determination of the passage of a correct key, provided that this current is limited correctly so that the recording of errors cannot occur. In accordance with the invention as disclosed in the parent application, this problem is resolved by a symmetrical operation of the microprocessor, with auto-verification of the write operations.
More particularly, data is always written in the memory whether the access key is correct or incorrect. The data carrier thus consumes the same amount of current whether the requested operation is authorized or unauthorized, and the would-be defrauder cannot therefore be informed of the results of the actions he undertakes.
During a writing operation, the device as briefly described above is effective and efficient, since the memory is modified in any case. Symmetry is ensured by using an error bit or an associated bit in the word to be written.
During a read operation, if it is desired to protect access with a key, it is necessary first of all to write in a memory zone reserved for this purpose.
In this way all the accesses are stored at the level of an access memory, which is an advantage when it is a matter of limiting the number of protected read operations.
On the other hand, this operation is penalizing at the level of the memory since it requires a relatively large space in memory.
Furthermore, it is inapplicable when the access should be protected during reading for an indefinite time as long as the working memory is not full.
In accordance with the present invention, this problem is resolved by adding to the storage devices described in the parent application an access memory and an error memory, both of the electrically erasable read only memory type.